Add example to read quadrature encoder in python with the PIO module

pio/pio_qei.py

@@ -0,0 +1,148 @@
+# Example using PIO to read a quadrature encoder
+#
+# Demonstrates:
+#   - PIO reading 2 pin
+# How to force a program to start at 0 by putting nop()
+# instructions at the end of the programm
+
+
+import time
+import rp2
+from machine import Pin
+from rp2 import PIO
+
+
+@rp2.asm_pio(in_shiftdir=PIO.SHIFT_LEFT)
+def QEI_prog():
+    #
+    # Copyright (c) 2021 pmarques-dev @ github
+    #
+    # SPDX-License-Identifier: BSD-3-Clause
+    #
+
+    #.program quadrature_encoder
+
+    # this code must be loaded into address 0, but at 29 instructions, it probably
+    # wouldn't be able to share space with other programs anyway
+    #.origin 0
+
+
+    # the code works by running a loop that continuously shifts the 2 phase pins into
+    # ISR and looks at the lower 4 bits to do a computed jump to an instruction that
+    # does the proper "do nothing" | "increment" | "decrement" action for that pin
+    # state change (or no change)
+
+    # ISR holds the last state of the 2 pins during most of the code. The Y register
+    # keeps the current encoder count and is incremented / decremented according to
+    # the steps sampled
+
+    # writing any non zero value to the TX FIFO makes the state machine push the
+    # current count to RX FIFO between 6 to 18 clocks afterwards. The worst case
+    # sampling loop takes 14 cycles, so this program is able to read step rates up
+    # to sysclk / 14  (e.g., sysclk 125MHz, max step rate = 8.9 Msteps/sec)
+
+
+    # 00 state
+    
+    jmp("update")	# read 00
+    jmp("decrement")	# read 01
+    jmp("increment")	# read 10
+    jmp("update")	# read 11
+
+    # 01 state
+    jmp("increment")	# read 00
+    jmp("update")	# read 01
+    jmp("update")	# read 10
+    jmp("decrement")	# read 11
+
+    # 10 state
+    jmp("decrement")	# read 00
+    jmp("update")	# read 01
+    jmp("update")	# read 10
+    jmp("increment")	# read 11
+
+    # to reduce code size, the last 2 states are implemented in place and become the
+    # target for the other jumps
+
+    # 11 state
+    jmp("update")	# read 00
+    jmp("increment")	# read 01
+    label("decrement")
+    # note: the target of this instruction must be the next address, so that
+    # the effect of the instruction does not depend on the value of Y. The
+    # same is true for the "JMP X--" below. Basically "JMP Y--, <next addr>"
+    # is just a pure "decrement Y" instruction, with no other side effects
+    jmp(y_dec, "update")	# read 10
+
+    # this is where the main loop starts
+    wrap_target()
+    label("update")
+    # we start by checking the TX FIFO to see if the main code is asking for
+    # the current count after the PULL noblock, OSR will have either 0 if
+    # there was nothing or the value that was there
+    set(x, 0)
+    pull(noblock)
+
+    # since there are not many free registers, and PULL is done into OSR, we
+    # have to do some juggling to avoid losing the state information and
+    # still place the values where we need them
+    mov(x, osr)
+    mov(osr, isr)
+
+    # the main code did not ask for the count, so just go to "sample_pins"
+    jmp(not_x, "sample_pins")
+
+    # if it did ask for the count, then we push it
+    mov(isr, y) # we trash ISR, but we already have a copy in OSR
+    push()
+
+    label("sample_pins")
+    # we shift into ISR the last state of the 2 input pins (now in OSR) and
+    # the new state of the 2 pins, thus producing the 4 bit target for the
+    # computed jump into the correct action for this state
+    mov(isr, null)
+    in_(osr, 2)
+    in_(pins, 2)
+    mov(pc, isr)
+
+    # the PIO does not have a increment instruction, so to do that we do a
+    # negate, decrement, negate sequence
+    label("increment")
+    mov(x, invert(y))
+    jmp(x_dec, "increment_cont")
+    label("increment_cont")
+    mov(y, invert(x))
+    wrap() # the .wrap here avoids one jump instruction and saves a cycle too
+    # Without these 3 instructions, the code wasn't working.
+    # did python put the start of the program somewhere else than at 0 ?
+    # With these 3 nop(), there is no choice, it should use the entire 32 bytes of memory
+    nop()
+    nop()
+    nop()
+
+class PIOQEI:
+    def __init__(self, sm_id, pin):
+        Pin(pin, Pin.IN)
+        Pin(pin+1, Pin.IN)
+
+        self.sm = rp2.StateMachine(sm_id, prog=QEI_prog, in_base=pin, in_shiftdir=PIO.SHIFT_LEFT)
+        self.sm.active(1)
+        
+
+    def get(self):
+        # Minimum value is -1 (completely turn off), 0 actually still produces narrow pulse
+        self.sm.put(1)
+        return self.sm.get()
+        
+
+# Create the StateMachine with the QEI program, reading on Pin(2) & Pin(3).
+qei1 = PIOQEI(1, 2)
+qei2 = PIOQEI(0, 11)
+
+while True:
+    print("count1:" + str(qei1.get()))
+    print("count2:" + str(qei2.get()))
+    time.sleep_ms(100)
+    
+
+